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In Robertson-Walker universe, light is emitted from a star with spatial coordinates $(r_s,\theta_s,\phi_s)$. It travels radially inwards and is received by an observer situated at the origin $(r=0)$. Show that the ratio of the observed wavelength $\lambda$ to the proper wavelength $\lambda_0)$ is given by $$\lambda /\lambda_0=R(t_2)/R(t_1),$$ where $t_1$ is the time of emision, and $t_2$ is the time of reception.

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Doy you mean spatial coordinates? And what is $R(t)$? –  draks ... Sep 27 '12 at 15:03
    
What have you tried? Where did you get stuck? –  Neal Sep 27 '12 at 15:21
    
This might be better placed at phyiscs.stackexchange.com. –  joriki Sep 27 '12 at 15:43
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@draks From the context it looks like he means spherical coordinates. –  rschwieb Sep 27 '12 at 16:23
    
I think that in order to be able to answer the question, we should exactly know which coordinate representation of the metric he is talking about. –  Raskolnikov Oct 2 '12 at 15:31

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